As evaporators for the recovery of volatile components from liquids of high viscosities, there have conventionally been used evaporators with a number of heating tubes built therein, jacket-type batch evaporators and centrifugal falling-film evaporators.
Among such conventional evaporators, the centrifugal falling-film evaporators are believed to be most suitable for the recovery of volatile components from high-viscosity liquids as far as the present inventors are aware of, because they have inter alia such merits that (a) they can achieve high heat efficiencies upon evaporation of volatile components and (b) they require shorter retention time periods for liquids to be treated therein, whereby they are appropriate as apparatus for the recovery of volatile components susceptible to thermal decomposition; (c) they have structures suited for operation under reduced pressures, thereby permitting low-temperature evaporation of volatile components having high boiling points or which are liable to thermal decomposition; and (d) their heating surfaces have structures resistant to scale deposition and they hence permit long-term continuous operation without development of any substantial reduction of their efficiencies of heat conduction during their operation.
When the recovery of a volatile component from a liquid having a high viscosity is attempted by means of a centrifugal falling-film evaporator, an evaporation residue is generally formed as a tar-like matter which still contains the target component of the recovery, namely, the volatile component at a significant concentration. It is thus not preferred from the economical viewpoint to discard the evaporation residue as is. Due to the tar-like nature of the evaporation residue, its disposal is cumbersome and time-consuming and moreover, requires a variety of difficult precautions to prevent environmental contamination.
In order to achieve further recovery of the volatile component from the tar-like evaporation residue, it may be contemplated to treat it in an evaporator of a different type, generally, in a batch evaporator. As a matter of fact, such an attempt was also made by the present inventors. It was impossible to improve the recovery rate of the remaining volatile component to any significant extent even when the tar-like evaporation residue was charged in conventionally-known evaporators of various types. Hence, the evaporation residue still remained in a tar-like form even after treated treatment in such additional evaporators. The above-mentioned various problems remained unsolved.
The present inventors strived to find solutions to the above-mentioned problems, resulting in the development of evaporators according to this invention.